Graphene based structures and methods for shielding electromagnetic radiation

ABSTRACT

Electromagnetic interference shielding structures and methods of shielding an object form electromagnetic radiation at frequencies greater than a megahertz generally include providing doped graphene sheets about the object to be shielded. The doped graphene sheets have a dopant concentration that is effective to reflect and/or absorb the electromagnetic radiation.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation application of and claims priority toU.S. Application Ser. No. 13/523,178, filed on Jun. 14, 2012,incorporated herein by reference in its entirety.

BACKGROUND

The present invention generally relates to structures and methods forshielding electromagnetic waves using graphene, and more particularly,to methods and structures of doped graphene sheets configured to reflectand/or absorb the electromagnetic waves being emitted from aelectromagnetic wave generating source.

Emission of electromagnetic (EM) radiation at radio, microwave andterahertz frequencies is known to interfere with operation of electronicdevices and has been linked to various health hazards to exposedindividuals. For example, the World Health Organization has recentlyannounced that exposure to microwave radiation could increase the riskof brain cancer. Because of concerns such as these, EM radiation is aserious issue and attempts to provide various shielding materials anddevices have evolved. Most commonly used EM shields in use today arefabricated from metallic films, metallic grids, metallic foams, orpowders on glass or plastic substrates. One example is a shielded cable,which has electromagnetic shielding in the form of a wire meshsurrounding an inner core conductor. The shielding impedes the escape ofany signal from the core conductor, and also signals from being added tothe core conductor. Some cables have two separate coaxial screens, oneconnected at both ends, the other at one end only, to maximize shieldingof both electromagnetic and electrostatic fields. Another example is thedoor of a microwave oven, which typically has a metallic screen builtinto the window. From the perspective of microwaves (with wavelengths of12 cm) this screen in combination with the oven's metal housing providesa Faraday cage. Visible light, with wavelengths ranging between 400 nmand 700 nm, passes easily between the openings the metallic screenwhereas microwaves are contained within the oven itself.

Due to the inherent weight of metallic shields, the added weight can besignificant. Moreover, many of the currently available EM shields arenot transparent, which can be a significant disadvantage for manyapplications. Conventional transparent and conductive materials such asindium tin oxide (ITO) and zinc oxide (ZnO) have been applied totransparent substrates such as glass and plastics for EM shielding.However, the use of these types of transparent EM shields is fairlylimited in their use because the shielding effectiveness of thesematerials is generally low, the shield itself is typically inflexible,and these types of EM shields provide limited mechanical strength.Providing higher EM effectiveness with these types of materials requiresincreased thicknesses, which then affect transparency.

SUMMARY

According to an embodiment, a method for shielding an object fromelectromagnetic radiation at frequencies greater than a megahertzemitted from an electromagnetic source, comprises providing one or moregraphene sheets on or about the object, wherein at least one or more ofthe graphene sheets are doped with a dopant.

According to an embodiment, a method for shielding an object fromelectromagnetic radiation at frequencies greater than a megahertzemitted from an electromagnetic source comprises providing one or moregraphene sheets on or about the object, wherein at least one or more ofthe graphene sheets are doped with a dopant in amount effective toreflect and/or absorb the electromagnetic radiation.

Additional features and advantages are realized through the techniquesof the present invention. Other embodiments and aspects of the inventionare described in detail herein and are considered a part of the claimedinvention. For a better understanding of the invention with advantagesand features, refer to the description and to the drawings.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

The subject matter which is regarded as the invention is particularlypointed out and distinctly claimed in the claims at the conclusion ofthe specification. The forgoing and other features, and advantages ofthe invention are apparent from the following detailed description takenin conjunction with the accompanying drawings in which:

FIG. 1 illustrates electromagnetic shielding structure for an object tobe shielded from electromagnetic radiation, the structure includingindividually doped graphene sheets according to an embodiment.

FIG. 2 illustrates electromagnetic shielding structure for an object tobe shielded from electromagnetic radiation, the structure including adoped uppermost graphene sheet according to an embodiment.

DETAILED DESCRIPTION

Disclosed herein are electromagnetic shielding structures and methodsfor shielding electromagnetic radiation emitted from an electromagneticradiation source. The electromagnetic shield structures are generallyformed from one or more sheets of doped graphene.

Graphene is atomically thin and has a zero band gap. Its lineardispersion around the K (K′) point leads to constant interbandabsorption (from valence to conduction bands, about 2.3%) of verticalincidence light in a very broadband wavelength range. By doping thegraphene sheets, higher carrier absorption can be obtained as well hashigher transparency in the near infrared and visible wavelength rangesdue to Pauili blocking.

Disclosed herein are electromagnetic shielding structures and methodsfor shielding electromagnetic radiation emitted from an electromagneticradiation source. The electromagnetic shield structures are generallyformed from one or more sheets of doped graphene.

Graphene is a two dimensional allotrope of carbon atoms arranged in aplanar, hexagonal structure. It features useful electronic propertiesincluding bipolarity, high purity, high mobility, and high criticalcurrent density. Electron mobility values as high as 200,000 cm²/Vs atroom temperature have been reported.

Structurally, graphene has hybrid orbitals formed by sp2 hybridization.In the sp2 hybridization, the 2s orbital and two of the three 2porbitals mix to form three sp2 orbitals. The one remaining p-orbitalforms a pi-bond between the carbon atoms. Similar to the structure ofbenzene the structure of graphene has a conjugated ring of thep-orbitals which exhibits a stabilization that is stronger than would beexpected by the stabilization of conjugation alone, i.e., the graphenestructure is aromatic. Unlike other allotropes of carbon such asdiamond, amorphous carbon, carbon nanofoam, or fullerenes, graphene isnot an allotrope of carbon since the thickness of graphene is one atomiccarbon layer i.e., a sheet of graphene does not form a three dimensionalcrystal. However, multiple sheets of graphene may be stacked. A typicalgraphene “layer” may comprise a single sheet or multiple sheets ofgraphene, for example, between 1 sheet and 10 sheets.

Graphene has an unusual band structure in which conical electron andhole pockets meet only at the K-points of the Brillouin zone in momentumspace. The energy of the charge carriers, i.e., electrons or holes, hasa linear dependence on the momentum of the carriers. As a consequence,the carriers behave as relativistic Dirac-Fermions having an effectivemass of zero and moving at the effective speed of light of ceJf£106m/sec. Their relativistic quantum mechanical behavior is governed byDirac's equation. As a consequence, graphene sheets have a large carriermobility of up to 60,000 cm2/V-sec at 4K. At 300K, the carrier mobilityis about 15,000 cm2/V-sec. Also, quantum Hall effect has been observedin graphene sheets.

By doping the graphene sheets, higher carrier absorption can be obtainedas well has higher transparency in the near infrared and visiblewavelengths due to Pauili blocking.

Advantageously, the electromagnetic shield structures according to thepresent disclosure provide effective shielding by reflection and/orabsorption at a frequency range of about 1 megahertz to about a fewhundred gigahertz, which is a significant improvement over priorelectromagnetic shielding materials. Moreover, because graphene is a oneatom thick monolayer sheet formed of carbon atoms packed in a honeycombcrystalline lattice, wherein each carbon atom is bonded to threeadjacent carbon atoms via sp² bonding, the overall thickness required toprovide>40 decibel (dB) shielding effectiveness, for example, is on theorder of a few nanometers. Still further, shielding effectiveness isincreased by doping the graphene sheets. As such, the use of dopedgraphene sheet(s) provides minimal added weight to the object to beshielded, has broadband capabilities, and provides greater versatilityas a function of its doping. Moreover, graphene is generally recognizedfor its high mechanical strength and high stability. In contrast, priorelectromagnetic shield materials require an increased thickness toincrease shielding effectiveness. In the present disclosure, increasingthe level of doping for a given thickness of stacked graphite sheetsprovides increased shield effectiveness.

The graphene sheets can be made by any suitable process known in theart. For example, graphene can be formed by solid state graphitization,i.e., by sublimating silicon atoms from a surface of silicon carbidesurface such as a (001) surface. At about 1,150° C., a complex patternof a surface reconstruction begins to appear at an initial stage ofgraphitization. Typically, a higher temperature is needed to form agraphene layer.

Formation of a graphene layer on another material is known in the art.For example, single or several layers of graphene may be formed one asilicon carbide substrate by sublimation decomposition of a surfacelayer of a silicon carbide material.

U.S. Pat. No. 7,071,258 to Jang et al. and U.S. Pat. No. 6,869,581 toKishi et al. describe known properties and methods of forming graphenelayers, the contents of which are incorporated by reference. Further USPat. Application Publication No. 2006/00099750 to DeHeer et al. and U.S.Pat. No. 7,015,142 to DeHeer et al. describe methods of forming graphenelayer, the contents of which are incorporated by reference.

The graphene can be formed on a substrate as may be desired in someapplications. The particular substrate is not intended to be limited andmay even include the electromagnetic radiation source itself. In oneembodiment, the substrate si transparent. In other embodiments, thesubstrate is flexible. In still other embodiments, the substrate is bothflexible and transparent. Likewise, the shape of the substrate is notintended to be limited. For example, the substrate may have planarand/or curvilinear surfaces such as may be found in foils, plates,tubes, and the like. Moreover, the substrate material is not intended tobe limited. Suitable materials include plastics, metals, and the like.

By way of example only, chemical vapor deposition (CVD) onto a metal(i.e., foil) substrate can be used to form the graphene sheets. See, forexample, Li et al., “Large-Area Synthesis of High-Quality and UniformGraphene Films on Copper Foils,” Science, 324, pgs. 1312-1314 (2009)(hereinafter “Li”) and Kim et al., “Large-Scale Pattern Growth ofGraphene Films for Stretchable Transparent Electrodes,” Nature, vol.457, pgs. 706-710 (2009) (hereinafter “Kim”), the contents of each ofwhich are incorporated by reference herein. Chemical exfoliation mayalso be used to form the graphene sheets. These techniques are known tothose of skill in the art and thus are not described further herein. Theas-prepared graphene sheets typically have a sheet resistance of fromabout 250 ohms per square (ohm/sq) to about 4,000 ohm/sq, depending onthe fabrication process.

Once the graphene sheets are formed, the sheets are deposited onto asubstrate using conventional lift-off techniques. In general, the sheetsare deposited one on top of another to form the film. Thus, by way ofexample only, the graphene film can comprise a stack of multiplegraphene sheets (also called layers). The term “substrate” is used togenerally refer to any suitable substrate on which one would want todeposit a graphene film. By way of example only, the substrate can be anobject to be shielded or may be a flexible film, which may optionally betransparent. The flexible film may then be applied to the object to beshielded.

The step of combining the doped graphene film with one or morestructural materials to form a composite material can be done using avariety of techniques known in the art that suitably preserve theintegrity of the graphene film. A wide variety of structural materialsare envisioned for use in the construction of the composite material. Inone embodiment, the structural materials may include essentially any lowconductive substrate or structure. For example, the structural materialmay include foams, honeycombs, glass fiber laminates, Kevlar fibercomposites, polymeric materials, or combinations thereof. Non-limitingexamples of suitable structural materials include polyurethanes,silicones, fluorosilicones, polycarbonates, ethylene vinyl acetates,acrylonitrile-butadiene-styrenes, polysulfones, acrylics, polyvinylchlorides, polyphenylene ethers, polystyrenes, polyamides, nylons,polyolefins, poly(ether ether ketones), polyimides, polyetherimides,polybutylene terephthalates, polyethylene terephthalates,fluoropolymers, polyesters, acetals, liquid crystal polymers,polymethylacrylates, polyphenylene oxides, polystyrenes, epoxies,phenolics, chlorosulfonates, polybutadienes, buna-N, butyls, neoprenes,nitriles, polyisoprenes, natural rubbers, and copolymer rubbers such asstyrene-isoprene-styrenes, styrene-butadiene-styrenes,ethylene-propylenes, ethylene-propylene-diene monomers (EPDM),nitrile-butadienes, and styrene-butadienes (SBR), and copolymers andblends thereof. Any of the forgoing materials may be used unfoarned or,if required by the application, blown or otherwise chemically orphysically processed into open or closed cell foam.

Likewise, the graphene films as described herein can be disposeddirectly onto the device to be protected electromagnetic radiation so asto encapsulate and/or enclose the device. The device may be virtuallyany device that includes an electronic circuit, non-limiting examples ofwhich include computers, mobile and landline telephones, televisions,radios, personal digital assistants, digital music players, medicalinstruments, automotive vehicles, aircraft, and satellites.

It should be apparent that using no more than routine experimentation,one skilled in the art can select structural materials for use with thegraphene film, based on properties such as operating temperature,hardness, chemical compatibility, resiliency, compliancy,compression-deflection, compression set, flexibility, ability to recoverafter deformation, modulus, tensile strength, elongation, forcedefection, flammability, or any other chemical or physical property.

In one embodiment shown in FIG. 1, the electromagnetic shield structure10 for shielding an object 12 from electromagnetic radiation includesone or more graphene sheets 14 ¹, 14 ², . . . 14 ^(n) are transferred tothe object to be shielded. Each individual graphene sheet is doped witha dopant 15 to enhance shielding effectiveness and transparency in thevisible range. In one embodiment, the graphene sheet is doped with ap-dopant such that electrons flow out of the graphene, therebyincreasing the work function of the graphene layer. Optionally, the oneor more graphene sheets are transferred to a flexible substrate 16. Inone embodiment, the flexible substrate is transparent to radiation inthe visible wavelength range. The number of graphene sheets utilizedwill vary depending on the intended application.

The number of graphene sheets utilized will vary depending on theintended application. For example, the graphene can be used as a singlelayer or in a multilayer configuration as described above. As such, thegraphene layer can have a thickness of about 1 nanometer to about 100nanometers, a thickness of about 10 nm to about 80 nm in otherembodiments, and a thickness of up to about 100 nm in still otherembodiments.

In another embodiment shown in FIG. 2, the electromagnetic shieldstructure 20 for shielding an object 22 from electromagnetic radiationincludes one or more graphene sheets 24 ¹, 24 ², . . . 24 ^(n) aretransferred to the object to be shielded. Doping is performed on thetransferred sheets with a dopant 25 after all of the graphene sheetshave been transferred, i.e., doping is performed on the stack.Optionally, the one or more graphene sheets are transferred to aflexible substrate 6. In one embodiment, the flexible substrate istransparent to radiation in the visible wavelength range. The number ofgraphene sheets utilized will vary depending on the intendedapplication.

As discussed above, the graphene film is doped. As used herein, the termdoped refers to an amount of dopant used to effect a dopingconcentration (n) in the graphene sheet that is reflective. By way ofexample, the dopant concentration (n) is highly doped to effectreflection and is greater than 1e10¹³ cm⁻². In other embodiments, thedopant concentration is effective to absorb the electromagneticradiation. By way of example, the dopant concentration (n) is moderatelydoped at 1e1013 cm⁻²>n>1e10¹² cm⁻². In other embodiments, the dopantconcentration (n) is low doped at 1e10¹² cm⁻²>n>0 cm⁻².

The dopants may be applied as a solution and/or as a vapor. By way ofexample, the graphene sheets are added to a solution of the dopant attemperatures of about room temperature to about 120° C. with agitationfor about an hour to several days. At the end of this process, thegraphene sheets are now highly doped. The residual doping agents areremoved via separation technologies (filtration wash, centrifugation,cross-flow filtration).

Examples of suitable dopants for increasing shielding effectivenessinclude oxidizing dopant such as, without limitation, hydrobromic acid,hydroiodic acid, nitric acid, sulfuric acid, oleum, hydrochloric acid,citric acid, oxalic acid, or metal salts, examples of which include, butnot limited to, gold chloride, silver nitrate, and the like. Exposingthe graphene film to the dopant solution and/or vapor shifts thegraphene Fermi level further away from the Dirac point, leading to alarge increase in the conductivity and reduction of the sheet resistancewithout interrupting the conjugated network of the graphene sheet.

The shield effectiveness (SE) in dB is expressed by the followingequations: SE=20 log(Ei/Et); SE=10 log (Pi/Pt), wherein E is the fieldstrength in V/m, P is the field strength in W/m², i is the incident wavefield, and t is the conduction field. In the present disclosure, theshielding effectiveness (SE) of the electromagnetic interferenceshielding structure according to the present disclosure is at least 30dB or more, and in other embodiments greater than 40 dB or more, whenthe frequency of the electromagnetic waves is greater than 1 MHz.

The terminology used herein is for the purpose of describing particularembodiments only and is not intended to be limiting of the invention. Asused herein, the singular forms “a”, “an” and “the” are intended toinclude the plural forms as well, unless the context clearly indicatesotherwise. It will be further understood that the terms “comprises”and/or “comprising,” when used in this specification, specify thepresence of stated features, integers, steps, operations, elements,and/or components, but do not preclude the presence or addition of oneor more other features, integers, steps, operations, elements,components, and/or groups thereof.

The corresponding structures, materials, acts, and equivalents of allmeans or step plus function elements in the claims below are intended toinclude any structure, material, or act for performing the function incombination with other claimed elements as specifically claimed. Thedescription of the present invention has been presented for purposes ofillustration and description, but is not intended to be exhaustive orlimited to the invention in the form disclosed. Many modifications andvariations will be apparent to those of ordinary skill in the artwithout departing from the scope and spirit of the invention. Theembodiment was chosen and described in order to best explain theprinciples of the invention and the practical application, and to enableothers of ordinary skill in the art to understand the invention forvarious embodiments with various modifications as are suited to theparticular use contemplated.

What is claimed is:
 1. A method for shielding an object fromelectromagnetic radiation at frequencies greater than a megahertzemitted from an electromagnetic source, comprising: disposing directlyonto the object one or more graphene sheets stackedly arranged and indirect contact with one another, wherein the object includes anelectronic circuit and is encapsulated with the one or more graphenesheet, and wherein the one or more graphene sheets are not a componentof the electronic circuit; and doping the one or more graphene sheets,wherein at least one or more of the graphene sheets are doped with adopant having a dopant concentration in an amount effective to reflectelectromagnetic radiation at frequencies greater than 1 megahertz,wherein the dopant is selected from a metal salt selected from the groupconsisting of gold chloride and silver nitrate.
 2. The method of claim1, further comprising reflecting the electromagnetic radiation, whereinthe dopant concentration (n) is greater than 1e10¹³ cm⁻².
 3. The methodof claim 2, where the one or more doped graphene sheets are transparentto electromagnetic radiation within the visible spectrum.
 4. The methodof claim 1, further comprising absorbing the electromagnetic radiation,wherein the dopant concentration (n) is greater than 1e10¹³cm⁻²>n>1e10¹² cm⁻².
 5. The method of claim 1, further comprisingabsorbing the electromagnetic radiation, wherein the dopantconcentration (n) is greater than 1e10¹² cm⁻²>n>0 cm⁻².
 6. The method ofclaim 1, wherein providing one or more graphene sheets comprisestransferring a first graphene sheet to the object; doping the firstgraphene sheet to form a doped graphene sheet; transferring at least oneadditional graphene sheet to the first doped graphene sheet; and dopingthe at least one additional graphene sheet; wherein the process isrepeated until a desired thickness is obtained.
 7. The method of claim1, further comprising transferring the first graphene sheet to aflexible substrate, wherein the flexible substrate is disposed on orabout the object.
 8. The method of claim 1, wherein the object comprisescurvilinear surfaces.
 9. The method of claim 1, wherein the dopant isselected from the group consisting of inorganic acids and metal salts.10. The method of claim 9, wherein the metal salt is gold chloride. 11.The method of claim 9, wherein the inorganic acids are selected from thegroup consisting of hydrobromic acid, hydroiodic acid, nitric acid,sulfuric acid, oleum, hydrochloric acid, citric acid, and oxalic acid.12. The method of claim 1, wherein the one or more graphene sheets areformed by chemical vapor deposition.